1. Field of the Invention
The invention is generally related to the area of optical components. In particular, the invention is related to MEMS switches (Micro Electro Mechanical Systems) and method for controlling/monitoring switching statuses of one or more MEMS switches.
2. The Background of Related Art
Optical interconnections can be done in two ways: i) Optoelectronic, where the optical data are first converted into electronic signal, then cross-connection is accomplished by electronic circuit. The electronic output is then converted back into optical signal; ii) Direct cross-connection of optical channels. This optical (photonic domain) interconnection where the signal does not need to be converted back and forth is known as “all optical switching.”
Various efforts have been spent on developing faster electronics to process higher data bit rates and novel optical networks components to increase the information carrying capacity of the optical networks. As the data bit rates increase, it becomes increasingly difficult to implement electronic switching solutions. It is now known that the information carrying laser beams should be dealt with at the optical level.
Recently a class of MEMS switches has been deployed as optical switches. These MEMS devices typically are small in size and may be integrated with other electrical circuits on a common substrate (e.g., silicon substrate). As a result, the MEMS devices are being used in numerous applications such as optical switching and optical attenuators in optical communications. Generally, an MEMS optical switch includes a small mirror that can be extended and retracted in various positions. In one instance, the mirror can be respectively extended to interpose between optical channels such that an optical beam traveled in a channel is reflected. In another instance, the mirror can be respectively retracted to be away from the optical channel such that optical beam traveled in the channel passes through without being reflected. Accordingly, depending on the position of the mirror, the optical beam can be switched into different optical channels.
In operation, an optical beam is routed by a small mirror which is driven by an electrostatic force, or others, such as, magneto-static, or thermal. Because they allow mass manufacturing of accurate miniaturized devices using materials and processes that have been proven for their stability, precision, and reliability, the silicon-based MEMS switches have been proven to be the technology of choice for optical communications. However, there are two major drawbacks in MEMS switches that hinder their inclusive replacement of the traditional opto-mechanical switches: (1) guaranteed switching action, and (2) potential sticking of actuator (stiction adhesion). The fundamental requirements for a fiber optic switch are that, when a switch command is given, it not only executes the command by applying a power source to its actuation mechanism, it can also (1) provide a feedback signal for assuring the mechanical position change of the mirror, and (2) reduce the opportunity of stiction that affects its performance reliability.
For a MEMS switch, the assessment of its switching position is difficult, due to its miniature size and its integrated manufacturing process. In U.S. Pat. No. 6,301,402, an extra light source emitting light at an ‘out-of-band’ light frequency modulated at low frequency was employed for the purpose of detection switching mirror on/off state. To facilitating the detection of the switching mirror on/off state, other extra components are needed, such as 1×N switch, a tap coupler, a PIN detector, and a demodulator, in addition to electronic control and feedback circuitry. Not only does this approach add cost and complication, increases insertion loss, polarization dependent loss (PDL), and wavelength-dependent loss (WDL), it also adds noise to the transmitting optical signal.
U.S. Pat. No. 6,301,402 and U.S. Pat. No. 6,519,383 both shows that specially dedicated mirror arrays are added in addition to the actual mirrors used for switching. These extra mirrors are manufactured together with the switching mirrors for the purpose of testing the proper positions of the switching mirrors. It can be appreciated that the switching systems as disclosed are not only expensive but also considerably reduces the manufacturing yield.
U.S. Pat. No. 6,459,524 also discloses that an additional pair of electrodes, besides the comb drive, is employed for the purpose of sending an electrical signal out when the two electrodes are in contact when the move of comb drive reach its final destination. Obviously, this method requires additional MEMS components being manufactured, which requires special wafer process, such as vertical metallization of these components, together with the required mirror or actuation driving mechanism.
Stiction adhesion can be very problematic in electrostatically-driven MEMS actuators, it degrade MEMS device reliability. To solve this problem, U.S. patent 2002/0167713 A1 utilizes a discharge system upon pull-in of moveable structure, this adds complexity of device manufacture.
There thus is a need for MEMS switches that guarantee switching actions with position indication and minimized potential stiction adhesion without much additional cost to the device manufacture.